The present invention relates generally to the field of wireless data communication networks and, in particular, to an improved, low power wireless radio system linked to a GPS receiver for increasing effective communication range without increasing output power of the transmitted signals.
Low power wireless data communications networks are used in a number of environments in which it is either impractical to run cabling for the local area network to fixed terminal sites, or where there is a need for mobile terminals that are in communication with the network. Examples of the former include rail yards, and other operations spread over a large area. In the latter category falls a common use of such systems, normally warehouses for stocking goods for shipment or accepting parts to be used in a manufacturing process. A typical system includes multiple base stations inter-connected by cabling to a network controller that processes the data received over the network and controls the transmission of data back to remote terminals through the base stations. A typical arrangement is one in which the network controller communicates with the host via serial data links employing twisted pair cabling.
The base stations communicate via wireless radio frequency link with a plurality of remote terminals that are typically either hand held or mounted on a vehicle such as a fork lift, cart or truck. Typical terminals include keyboards and displays and some are equipped with scanning bar code readers. The networks may be used as part of any underlying system for operation of the facility in which they are used, such as assistance in taking inventory, filling orders, directing employees as to the placement of inbound products in the warehouse space and the like. Information from the remote terminals is normally entered via key pad or bar code reader. It may indicate that an assigned task has been completed, that problems have been encountered such as the absence of sufficient goods of a particular type to fill a particular order, and so forth.
Any application that the user wishes to implement, that can be handled by the host computer, can be implemented by using the wireless data communications network. For example, the host computer can determine minimum length paths for filling particular orders and transmit to a remote terminal located on a transportation device in a warehouse the sequence in which items should be selected for filling a particular order. The systems are also very useful in environments in which just in time parts inventory systems are implemented for prioritizing movement of materials from a receiving dock to locations in a warehouse or manufacturing facility at which they are needed.
Another application where it is desirable to use mobile terminals that are in communication with a centralized computer network is in the area of geodetic surveying. Modern techniques for performing geodetic surveying typically include mobile geodetic survey stations which utilize GPS or other satellite positioning signals (SATPS) for determining precise coordinates of ground locations on the earth's surface. However, in many instances, the GPS receivers utilized by the geodetic survey stations are not able to provide precise coordinate positions of sufficient accuracy.
For this reason, in order to improve the measurable accuracy of the GPS coordinates at a particular ground location, the geodetic survey station at the ground location communicates by wireless radio transmission with a base reference station having precisely known coordinates. The information transmitted by the base station is used by the remote survey station to correct for coordinate measurement error inherent in the mobile GPS receiver. In this way, by combining the GPS positional information from the GPS receiver with the correctional information transmitted by the base reference station, precise coordinates of the ground location at the mobile survey station may be quickly and accurately determined. Additionally, if desirable, the base reference station may upload information transmitted from the mobile station for use in developing a resource or GIS map of the surveyed area.
Largely by regulatory constraint in the United States of America, wireless, mobile communication systems are typically operated at a low power level since the authorized radio transmissions for such applications are in shared portions of the electromagnetic spectrum under the regulatory scheme implemented by the United States Federal Communications Commission (FCC). Since 1985, the FCC has approved the use of low power non-license Business Radio Systems regulated under Sub-part D of Part 15 of Title 47 of the Code of Federal Regulations (CFR). Three bands are authorized for such use: 902 through 928 MHz, 2400 through 2483.5 MHz, and 5725 through 5850 MHz. According to 47 CFR Section 15.247, for example, there are a number of constraints on the operations of such systems including a maximum radiated power of 1 watt, and a limitation on spectrum spreading techniques to direct sequencing and frequency hopping. A copy of 47 CFR Section 15.247 is provided as Appendix A of this application.
In view of the above cited regulatory constraints and common sense, it is desirable for the radio frequency networks described above to utilize low radiated power. Warehouse facilities are often concentrated in the same geographic area and a high level of radiated power increases the probability of intersystem interference. For the devices that operate in the shared portions of the electromagnetic spectrum, there is also a need to minimize the radiated power to avoid interference with other devices operating on the same frequencies. Additionally, a system that is designed for relatively high power output would require high radiated power from its roaming terminals, which are typically battery operated. This situation would lead to requirements for larger, heavier batteries or shorter battery life.
One advantage of operating a radio device in compliance with the FCC regulations is that the user is not required to obtain an FCC license in order to operate such radio devices. For example, radio devices operating at 902 MHz and broadcasting at a maximum radiated power of 1 watt do not require an FCC licensed user for operation. However, the same radio device operating at a radiated power of over 1 watt requires a FCC licensed user to operate the radio. Thus, radio devices using a maximum radiated power of 1 watt are made accessible to virtually an unlimited number of users. Unfortunately, by limiting the maximum radiated power of the radio device to 1 watt, the effective communication range of that radio device is also limited since the power of the output signal is directly proportional to the communication range of the radio device.
In applications such as geodetic surveying, however, large areas of land are typically required to be surveyed, encompassing tens or even hundreds of square miles. It is therefore desirable for each of the radio devices at each of the respective remote survey stations to be able to extend its effective communicating range within the wireless network as far as possible while simultaneously complying with FCC regulations such as those described above.
It is therefore an object of the present invention to provide a wireless radio communication network including at least one radio device which can be operated by any user (without requiring an FCC license), and which provides for an extended range of communication over that of similar-type conventional radio devices. A further object of the present invention is to provide the wireless radio communication device having improved sensitivity and accuracy for facilitating transmission and/or reception of modulated carrier frequency signals.